"The president directed the Department of Defense to carry out the intercept" after concluding that it would help prevent loss of life from the uncontrolled descent, said Deputy National Security Adviser James Jeffrey.

Jeffrey and other senior officials said the risk posed by an estimated 1,000 pounds of hydrazine, a toxic propellant, aboard the schoolbus-sized satellite was a key factor in the decision.

They denied that it was driven by the desire to protect the highly classified satellite's secrets, or that the shoot-down was intended to demonstrate a US anti-satellite defense capability.

China drew worldwide protests after it shot down a weather satellite in low Earth orbit January 2007.

The United States has never shot down a spacecraft in space before, but its missile defense system is designed to intercept incoming warheads in space.

"Our objective here was to reduce the risk. Could we reduce the risk to space platforms, to airborne platforms, and to terrestrial platforms -- the earth, cities, people, etcetera?" said General James Cartwright, vice chairman of the Joint Chiefs of Staff.

A US Aegis warship will fire a single modified SM-3 missile at the spy satellite in hopes of scoring a direct hit on a tank carrying the hydrazine, Cartwright, the vice chairman of the Joint Chiefs of Staff.

He said the plan is to hit the satellite at a point in its orbit where any debris will quickly fall out of orbit and re-enter the atmosphere over the ocean.

"And that's our objective: get rid of the hydrazine and have this fall in the ocean," Cartwright said.

The soonest that the intercept will take place is in three or four days, but the window will remain open for seven or eight days, Cartright said.

Three Aegis warships will be on station with two back-up missiles in case there is a problem with the launch of the first missile.

"We will have radars and space sensors pointed at the area so that we have some sense of whether we were successful or not," he said.

"In the case that we're not successful with the first shot, we'll reassess," he said.

But as the satellite moves across the Earth, the chances increase that an intercept will bring debris down over land, Cartwright said, adding "we're not going to shoot if that's the case."

"What we're looking for is to catch it here very close to the earth's surface. What we're shooting for, nominally, is about 130 miles (210 kilometers) up," he said.

He said about half the debris will come down in the first two revolutions if the intercept is successful, but it could talke longer than a month for some of the smaller debris to come down.

"But it's a very finite period of time that we can manage, and it's in an area where we don't have satellites manned or unmarked; in other words, down very low," he said.

NASA administrator Michael Griffin said the space shuttle Atlantis will have finished its space mission and have landed before the intercept.

"This missile is designed, of course, for other missions, but we concluded it could be reconfigured, both the missile and other systems related to it, on a one-time reversible basis to do the shot," he said.

Asked why this intercept was any different than the Chinese anti-satellite test, Cartwright said the United States was notifying the international community beforehand and was conducting the intercept near the edge of space.

Jeffrey said the Chinese test was conducted against a satellite in a circular orbit at around 530 miles (850 kilometers) of altitude, creating a debris field that could remain for decades over a large swathe of orbital space.

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